A typical aircraft auxiliary power unit (APU) consists of a turbine engine, a dc starter/clutch, and a 400 Hz electrical generator. It is desirable to combine the starter function and the generator function in order to reduce the number of auxiliary power unit accessories. A common approach to this problem is to drive the 400 Hz electrical generator as a motor to start the auxiliary power unit.
One such system utilizing this approach is disclosed in Hoffman et al. U.S. Pat. No. 4,093,869. This patent discloses generating apparatus in the form of a pilot excitor, a main excitor, and a main generator all of which are coupled to a prime mover. In the generating mode, the pilot excitor provides field current for the main excitor, which in turn generates field current for the main generator. When it is desired to utilize the generating apparatus to start the prime mover, a quadrature axis winding disposed about the stator of the main excitor is energized by a single phase alternating current. A corresponding alternating current is induced by transformer action within the armature of the excitor, and is utilized to supply main generator field current. At the same time, polyphase alternating current excitation is applied to the armature of the main generator to establish a rotating magnetic field which interacts with the stator magnetic field set up by the field current in the main generator. This interaction produces a mechanical turning force which is utilized to start the prime mover.
Messenger U.S. Pat. No. 3,908,161 discloses a generating system wherein a three-phase altering voltage is applied to a set of excitor field windings which are connected in wye configuration. The excitor operates as a rotating transformer, with the armature current of the excitor being rectified and applied to the field winding of the main generator to cause the main generator to operate as a motor, and thereby start a prime mover. Once the prime mover has been started, the rotating transformer is converted back to a brushless excitor by connecting the field windings in series and applying the dc voltage thereto from the output of a permanent magnet generator, or PMG. Another patent which discloses a system similar to the Messenger device is Lafuze U.S. Pat. No. 3,902,073.
The systems described above suffer from disadvantages. The Hoffman et al. system requires that a separate additional quadrature axis winding be disposed in the excitor stator, such winding being utilized only when the system is operating in the starting mode. The Messenger and Lafuze patents require that the excitor stator windings be switched from a series connection to a wye connection when it is desired to utilize the main generator to start the prime mover. Further, each system described above requires an external source of ac power, which may not be available on the aircraft. Typically, however, only a 28 volt dc battery is available when a start is required.
A device which eliminates the necessity for a starter motor and allows start power to be delivered from the battery is disclosed in Glennon U.S. Pat. No. 4,330,743 which is assigned to the assignee of the instant application. A reversible ac to dc converter receives external dc power and supplies ac power to drive an alternator as a motor to start a prime mover through a torque converter connected thereto.
The system requires sophisticated current control circuits to handle the large currents that must be delivered to the alternator to cause it to act as a motor. Furthermore, in the event that the alternator is a wound field machine, the field windings must be switched to a different configuration to allow it to operate as an induction motor.
A further system which allows auxiliary power unit start from a low voltage dc supply is described in Markunas U.S. Pat. No. 5,036,267 and Rozman et al. U.S. Pat. No. 5,055,764, which are also assigned to the assignee of the instant invention. This system requires an excitation inverter as well as a main inverter to operate the synchronous generator as a brushless dc motor to allow the machine to start the auxiliary power unit. Furthermore, an autotransformer is required to accelerate the machine to 100 percent operating speed as the voltage supplied through the main inverter is insufficient.
A further system which eliminates the necessity for an excitation inverter is described in Mehl et al. U.S. Pat. No. 4,481,459, which is assigned to the assignee of the instant invention. This system allows for motor start capability from a low voltage dc source through a dc to dc converter which provides input to an inverter which drives the permanent magnet generator as a brushless dc motor. Once the generator attains a certain operating speed through excitation of the permanent magnet generator, the power supply to the permanent magnet generator is removed, and the main generator is supplied power from a motor drive to cause the generator to act as a synchronous motor. A torque converter is then commanded to transfer motive power from the generator to start the prime mover.